1. Field of the Invention
This application concerns a culture medium for insect cells.
2. Brief Description of the Art
The baculovirus expression system using insect cells has become an important tool for production of recombinant proteins for several reasons. First, the expression levels are often very high compared to those obtained in other animal cells, such as CHO-cells. Second, proteins produced using this system are often biologically active due to the insect cells' ability to perform post-translational modifications, folding and assembly of most proteins. Third, the time from cloning to production of the protein is short compared to the time needed to construct a stably transformed animal cell line. Fourth, cell death inevitably follows virus infection of insect cells. This can be an advantage over other expression systems because it may permit better expression of cytotoxic, regulatory or essential cellular genes.
Foreign proteins are produced during a lytic infection of insect cells with a recombinant virus. Baculoviruses contain a very late hyper-expressed gene, polyhedrin, which is not essential for viral replication. Placing a gene under control of the polyhedrin promoter allows the production of large quantities of a recombinant protein. The strategy for production of a protein involves three distinct stages:
(a) growing the insect cells from mid to late growth phase; PA1 (b) infecting the cells with virus containing a gene encoding the protein of interest; and PA1 (c) harvesting and purification of the protein product.
Insect cells are conventionally cultured in complex media containing inorganic salts and sometimes organic salts, amino acids, sugars, vitamins, trace elements, lipids and protein hydrolysates. For example one commercially available medium, called, IPL-41, is available from a number of suppliers. Apart from these components, serum or yeast extract has conventionally been added to provide undefined but essential nutrients. The amino acid glutamine has hitherto been assumed to be essential for cultured insect cells (Wang, M-Y et al Biotechnol. Prog. 1993, 9, 355-361; Kamen, A. A. et al Biotechnol. Bioeng. 1991, 38, 619-628;Wang, M-Y et al Biotechnol. Prog. 1993 in press), as it is for most cultured mammalian cell lines.